Adds tests for the change to make extension members accessible even
for prefixed imports. Also tests that it is an error to import a
library with extensions as deferred without hiding the extensions.
Change-Id: I3a81c560694d34c8adbb4cc86ca9e6b8a129ae56
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/124589
Commit-Queue: Leaf Petersen <leafp@google.com>
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Leaf Petersen